Radio frequency coupling system



Nov. 10, 1942. J REID RADIO FREQUENCY COUPLING SYSTEM Filed Dec. 18, 1940 s Sheets-Sheet 4 n n w m R Jr G L M L. M A

l l BROADCAST MING RAIKfiI GAIN 1400 INV EN 1 OR.

- FREQUENCY (KC) F9 JOHN D. Rip BY QM d M/ ATT NEY Nov. 10, 1942 J. D. REID RADIO FREQUENCY COUPLING SYSTEM Filed Dec; 18, 1940 3 Sheets-Sheet 2 INVENTOR.

' JOHN D. RUB

Nov. 10, 1942. J, I 2,301,324.

RADIO FREQUENCY COUPLING SYSTEM Filed Dec/18, 1940 3 Sheets-Sheet s I I INVENTOR.

IQ. 5 JOHN D- RElD Patented Nov. 10, 1942 UNITED STATES PATENT OFFICE 2,301,324 I RADIO R Q E CY COUPLING SYSTEM John Drysdale Reid, Mount Healthy, Ohio,-as-

signor to The Crosley Corporation, Cincinnati, Ohio, a corporation of Ohio 7 Application December 18, 1940, Serial No. 370,688 7 13 Claims.

ceivers and certain aspects are of particular advantage in receivers of the superheterodyne type which use a combination of a built-in and an external antenna system. For facility of. explanation the invention will be described in its application to radio receivers, but its adaptation l5 to other uses will be apparent to persons skilled in the art.

Due, to a large extent, to the type of coupling systems at present in use, the performance of radio receivers in general leaves much to be desired with respect to sensitivity, selectivity, stray noise pick-up, and uniformity of proper response in relation to one or more of these conditions. With the superheterodyne type of receiver the signal-to-image ratio is often insufIi-' cient and interference with the desired signal frequently results. The present invention provides improved performance in these respects. To bring about such improved performance and to do this in a simple mannerwith a saving in 3 manufacturing cost, are important objects of the present invention.

A particular object of the invention is to provide a radio-frequency coupling system, im-

proved in one or more of the above respects, for

use in the input system of a radio receiver but which is also suitable for interstage use.

A further object is to provide a coupling circuit having the advantages indicated which may be utilized not only in receivers designed for the standard broadcast band but also in multi-band receivers, and which, in the latter case, requires a minimum of parts and a minimum of switching for band change purposes.

A further object is to provide a radio frequency the uniformity of gain over the tuning range independent of the capacitance of the antenna,

In the case of a built-in antenna the invention provides a low impedance built-in loop antenna which with suitable switching means may I be used to advantage in tuning throughout the lowest frequency band but which becomes a high impedance loop antenna constituting the entire secondary inductance for a higher frequency band. If desired the switching means may be so arranged that the loop serves as an electrostatic pick-up member for a still higher frequency band. This results in a considerable saving in the cost of receiver manufacture. Such a loop may in accordance with the invention, be of a substantially fixed nature thus simplifying the design and effecting further 'sav-' ings in cost. l The novel features that I consider characteristic" of my invention are set forth in the appended claims; the invention itself, however, both as to its organization and method of operation together with additional objects and advantages thereof will best be understood from'the l, following description of specific embodiments when read in conjunction withthe accompanying drawings, in which; n Figure 1 illustrates an exemplary coupling system applied to the input system of a single band radio receiver.

Figure 2 is a graph showing the characteristic impedance of the antenna system of Figure 1 with respect to frequency. a

Figure 3 is a graph illustrating the effect .of both direct andmagnetic coupling involved in Figure 1. Curve (a) shows the gain with magnetic coupling only; curve (b) with direct coupling only; curve 2) combined direct and magnetic coupling, and curve (d) shows how thegain rises yet remains uniform when the capacitance of the antenna is increased. 7,

Figure 4 shows an antenna and interstage cou-, pling system applied to the radio frequency circuits of a receiver incorporating a built-in loop antenna. I Figure 5 is a schematic circuit for a three-band receiver also incorporating a coupling system according to the invention for use with the lowest frequency band and direct capacity coupling for 0 use with the higher frequency bands.

Figure 6 illustrates a modified antenna circuit for a multiband receiver, in which a built-in loop I tenna the coupling system of the invention makes R f r i now t Figure 1 t input circuit cludes the antenna A, inductance L1, the capacitance C1, which shunts inductance L1, and the inductance L3 which must be small in relation to L1 and the lower end of which is connected to ground. The input circuit is completed through the effective antenna-to-ground capacitance C2. The output circuit includes the inductance L2 and the tuning condenser C3 and the inductance L3 the latter being electromagnetically uncoupled from L1 and L2 and being a common element to both circuits, directly coupling them together. C4 is a trimmer condenser of the adjustable type; Coils L1 and L2 are preferably electromagneticah ly coupled, as by a mutual inductance M. The first tube of the receiver, which may be a radio frequency amplifier is indicated at H] and its output may be taken from terminals H and I2 in a manner well known in the art. i

It will be observed that the respective circuits are electromagnetically coupled by reason of themutual inductance M, and direct coupled through inductance L3. netically coupled with either-L1 or L2.

As will hereinafter be more particularly described in connection with the succeeding figures, when a built-in antenna is'desired and the radio receiver is housed in a cabinet of relatively large,

tuned. At these frequencies L1 is equivalent to a. small capacitance in shunt with C1.

The mutual inductance between coils L1 and L2 is preferably such that it aids the direct coupling across inductance L3. It is also preferable to so wind and pole the inductances L1 and L2 that the capacity coupling between their high sides is a, minimum and that which exists is in aiding phase. This may be accomplished by winding L1 and L2 in the same direction and connecting the terminating end of L1 to the starting end of L2. In this manner freedom from shift of the, tuning of the output circuit for dif- InductanceL3 is not electromagdimensions such as a console or large tablemodel, the inductance L3 may be advantageously constructed in the form of a loop which has a small inductance and therefore-a low impedance. L3 may be conveniently constructed by passing one or two. turns of insulated wire around the back edge of the cabinet but it may be wound around the cabinet in other ways or even built in or concealed upon the cabinet walls; In the case of a small table model receiver where the dimensions of such a loop are so limited that a sufficiently large area for adequate signal pick-up cannot be obtained, it is possible to utilize the advantages of the invention for use with external antennae by designing the inductance L2 so that it, rather than the inductance L3 constitutes built-in pick-up means. In such case inductance L2 is made in the form of a loop and inductance L3 constructed in the form of .a small coil having a small inductance. H

The inductance L2 maybe chosen so that it constitutes the major portion of that required to tune the output circuit over the standard broadcast band, the remaining portion being derived from L3 which is small compared to L2, for example L3 may be of the order of of L2.

L1 is preferably a large inductance of the order of thirty-five times the inductance of L2 to which it is electromagnetically coupled as shown. It is is resonant far below any frequency to which the output circuit can be tuned, L3 offering a'negligible impedance to low frequencies by reason of its small inductance.

The inductance of L3 and the capacity-C1 may I be fixed so that the high frequency resonance of the circuit C2, C1, L3, will be far above the highest frequency to which the output circuit can be ferent capacitances across the input circuit, and greater attenuation of high frequencies above the tuning range of the output circuit, are obtained.

It will be seen that the arrangement just described has an input circuit which is resonant at two points. First, at a frequency far below any to which the output circuit can be tuned and, second, at a frequency far above any to which the output circuit can be tuned. This will be seen by reference to Fig. 2 in which the impedance of the input circuit is plotted against frequency. The first or low frequency resonance of the input circuit is at a, whereas the second or high frequency resonance thereof is at c; b and d denote anti-resonance points. The approximate position of the standard broadcast band of 530 to 1700 k. c. is indicated between frequencies e and f,

and it will be observed that the input circuit is capacitively reactive throughout this range of frequencies, since the impedance is falling as the frequency increases.

The combination of electromagnetic and direct coupling with the low and high frequency resonances of the input circuit results'in a uniform signal transfer or voltage gain from the antenna to the tuning condenser C3, across the output circuit. This uniformity manifests itself over the entire: tuning range of condenser C3. Since both resonances of the input system are far removed from the tuning range of the output circuit the system is inherently free from shift in tuning with the range of antennae-normally encountered in practice. That is, with antennae having capacitances of from 10 to 300 micromicrofarads,'

the antenna gain remains fiat over the band for any such antenna and the-tuning of condenser C3 gis substantiallyunaffected. The manner in which thegain remains substantially constant over the tuning range is apparent upon examination of Figure 3, curve (0), in which gain is plotted against frequency.

With the system above described, the selectivity of the receiver is maintained regardless of the antenna capacitance since the antenna resonances are maintained far removed from the tuning range. The gain increases with increase in ;,antenna capacitance. For instancecurve (d) in Figure3 indicates the approximate gain with an antenna having three times the capacitance of that used in plotting curve (0). The gain does not, however, vary more than 21 for a 10-1 variation in antenna capacitance. A comparison with the non-uniform gain of magnetic coupling alone or direct coupling alone may be made by referring to curvesfa) and (b) of Fig. 3. It may be explained that the curves of Fig. 3 were taken with the coils connected as previously described, L3 being shortedfor curve (a) and L1 being shorted for curve (b).

As a specific example of values that may be used With the circuitabove described for use with the broadcast band of approximately 530 to 1700 k. c., I have found the following to be satisfactory: L2=224 microhenries L3=13 microhenries L1=5890 microhenries M=392 microhenries L2 effective=l96.5 microhenries 03:10-430 micromicrofarads C1=55 micromicrofarads 02:10-300 micromicrofarads C4=2-20 micromicrofarads Distributed capacitances:

L1=8.7 micromicrofarads L2=8.2 micromicrofarads L3=23 micromicrofarads The foregoing values are such that the resonance frequency of C2 C1 L1 in the input circuit is between 110 and 245 k. c., frequencies .far' below the lowermost end of the tuning range of approximately 530 to 1700 k. c. The resonance frequency of C2 01 L3 in such circuit is between 5100 and 7800 k. c., frequencies far above the uppermost limit of said range. It is to be understood that these values are illustrative merely and that the advantages of the invention may be realized by utilizing values that will bring the resonance frequency of the first-mentioned circuit to the order of one-half or less of the lowermost frequency of the tuning range and of the second-mentioned circuit to the order of two times or more the highest frequency of said range.

In order to reduce the losses likely to result, all inductances and coil L2 in particular should be made with multi-strand wiresuch as Litz wire and may advantageously be wound upon a core of comminuted iron. This latter type of winding may also be found desirable for the purpose of increasing the degree of coupling between coils L1 and L2.

With a superheterodyne type of receiver there commonly occur spurious responses such as those caused by an image, or by the harmonics of the oscillator plus and minus the intermediate frequency. This is a serious problem in receiver design. The coupling system of the present invention affords increased attenuation of such responses over conventional designs since at high or image frequencies the voltage impressed on the output circuit by the input circuit is developed almost exclusively across L3, since the reactance of C1 is low compared to the reactance of L1 at high frequencies. The reactance of L2 is high compared with that of Ca at image frequencies and the distributed capacity of L2 tends to raise it still higher. Consequently the voltage developed across C3 due to the image signal is only a small part of that developed across L3. Additional capacity may be shunted across L2, for

instance the trimmer condenser C4, in order to Referring now to Figure 4, I have there shown a circuit in which the coupling system .of the .75

' cept for the addition of interstage coupling. In

Fig": 4, however, I have shown a blocking condenser C5 for the purpose of avoiding subjecting the tuning condenser C3 to a high D. C. potential anda shunt feed resistor R1 for the purpose of obtaining a proper operating potential on the plate of the amplifier I0. Resistor R1 is in such case made of sufiicient size to avoid serious loading of the output circuit due to the fact that it is' efiectively in shunt with the inductance L3 of the interstage circuit. L3 in the antenna circuit is shown as, and it preferably is, an internal loop-antenna of relatively large area of the type previously alluded to while L3 in th interstage amplifier is merely a small coil having the same inductance. When operated without an external antenna it is generally desirable to short-circuit L1 as by a strap 2| as indicated at the left of Fig. 4.

Resistances R2 and R3 are for the purpose of maintaining proper potentials on th grids of tubes l0 and 20, and C6, C7, Cs are by-pass conden'sers, and condensers C9 are blocking condensers for preventing direct current from flowing in the parts of th circuits indicated. These are conventional elements and do not form any part per se of the present invention.

In Fig. 5 I have shown the invention applied for use with a three-band radio receiver. The circuit illustrated is similar to Fig. 4. Switching means are provided so that appropriate circuits maybe made for the additional bands. The switch may conveniently take the form shown, in which switch 30 has a short arm 3| and a long arm 32, and is pivoted at 29 at which point the leads 31, 38 and 39 may be connected. Contacts 33, 34 and 35 may be provided for cooperation with the switch 30. Contact 34 is elongated somewhat so that the short arm 3| will cooperate therewith when such arm is in proper position.

Forifacility of explanation I will describe the switching means with respect only to the antenna circuit of Fig. 5. With the switch 30 in the position shown in Fig. 5 the inductance L2 is shorted through contact 34 and lead 36 and the inductance L3 forms the entire inductance in the output circuit to which the input circuit is coupled by capacitance C1. This capacitance may be made small to facilitate proper coupling. In-

ductance L3 and condenser C3 may be. so related circuit thus made may be used for short wave reception.

With the switch 30 turned counter-clockwise from the position shown in Fig. 5, the short is removed from inductance L2 and the same circuit is obtained as is shown in Fig. 4 for use in tuning over a lower frequency range such as the standard broadcast band. It will be noted that an additional capacitance C may be shunted across L3 in the interstage circuit in order to compensate for the capacitance inherent in loop L3.

circuit. L1 and L2 again illustrate the coupling transformer. as a built-in loop antenna and serves as a pickup member for all three bands. The inductance L3 constitutes a low impedance 100p for the lowest frequency band,a high impedance loop forthe middle frequency band and a capacitive antenna member for the highest frequency band.

The switching means 50 in Fig. 6 may be con-' veniently made by mounting a pair of partial sectors i and 52 of conductive metal on an insulat ing disc 53 mounted for rotation about pivot 54.

Appropriate leads from the respective circuitsare connected to predetermined contact l'B8l located in a circle and each being equidistant from pivot 54. Th switch has four operating positions for switching in, respectively, a push button control for the standard broadcast band, manual tuning for such band, an intermediate band such as a police band and a high frequency or short wave band.

With switch 56 in the position shown, the receiver is arranged for push button control. The input circuit may be traced from the external antenna connection A, through the parallel circuit C1, L1, inductance L3, contact '13, switch. member 52, elongated contact 19 and thence to ground. The output circuit is from ground,

through 39, 52, 18, L3, L2, junction 55, and con-- denser C9 to the grid of tube 15. Across the output circuit may be placed one of a number of The inductance L3 is constructed" preadjusted capacitances C60, C61, etc., only two of which are shown, by actuation of one of the push buttons 60, 6|, etc. This parallel branch runs from ground, through C'so for example, push button switch 66, contact 10, switch member 5|, contact l3 and lead 55 to the junction point 56. Condenser C4 is the trimmer condenser for the broadcast band manual tuning condenser C3 and may be preadjusted at the factory so that its capacitance does not detune the push button tuning circuits. The short wave tuning circuit L5 C5 is shorted with the switch in the position shown. The other contacts of the switch are open circuited.

If the switch 50 be now turned. clockwise one position the push button tuning circuits are discarded and the manual tuning condenser C3 is inserted across the output circuit. The other circuits are not changed except the short wave tuning circuit L5 C5 previously shorted is disconnected from ground.

If the switch be now moved a second step in a clockwise direction the trimmer C4 for the manual tuning condenser C3 is removed from its position across the output circuit and the secondary L2 shorted by means of switch member 5| making contact between contacts 13 and T5. The antenna-to-ground and grid-to-ground paths of the input and output circuits respectively remain otherwise unaffected since a path is maintained through L3, switch member 52 and contact 19.

However, tuning condenser C3 remains across'Ls which in this case constitutes the entire output and consequently its adjustment has negligible effect upon lower frequency bands.

Finally, if the switch is moved clockwise three steps from that shown in Fig. 6 the coil L2 remains shorted but short wave coil L5 with its F In Fig. 6 I have shown a modification of the circuit shown in Fig. 5, as applied to the antenna.

trimmer Cell]. parallel therewith is placed across the circuit. C3 remains in shunt with the output circuit. This enables the set to be used for short wave reception, the coil L3 serving as an electrostatic pick-up member since it is connected in circuit at one end only through contact '45. Contact 18 is disconnected from arm 52in this position. Due to the cut-out in switch member 52 there is no contact made between member 52 and contact 10 in this position.

Since member 52 contacts member 8! in this last position any desired auxiliary switching of a point to ground may be accomplished For instance, contact 8l may be connected to cathode as shown in order to short circuit resistance R4,

thereby reducing the bias potential on the grid of tube Hi. This increases the amplification of the tube I!) for short wave reception.

The low impedance loop antenna described if of simple construction well adapted to be placed inv or upon console model or large table model radio receivers by merely passing one or two turns therearound. This type of loop is of particular advantage in that it'has a symmetrical directional pattern presenting large angles over which the'signal strength is good and relatively small angles over which it is weak. This permits it to be more or less fixed with respect to the cabinet thus obviating the necessity of special mounting for rotation. With the conventional type of loop antenna consisting of a relatively large number of turns the directional pattern is non-symmetrical due to the antenna effect of the high end of the coil and the angle in which poor signal strength occurs is so large that a rotatable mounting is required.

Having thus described my invention, I claim:

14A radio frequency coupling system comprising an input circuit having a series capacitance and a tuned output circuit, the input circuit comprising aninduct'ance and a capacitance in parallel therewith, said inductance and capacitance being such that their resonant frequency is at least one-half the lowest frequency of said tuned output circuit, said output circuit comprising an inductance magnetically coupled to the first mentioned inductance and a separate inductance common to both circuits magnetically uncoupled to the other inductances, said separate in- .ductance having a low value such that it resonates with said parallel capacitance and said series capacitance at a frequency approximately twice the highest frequency of said tuned output circuit.

2. A radio frequency coupling system comprising an input circuit and an output circuit, means magnetically coupling said circuits, means independent of magnetic coupling for directly coupling the same and means for tuning the output circuit, said independent means consisting of a loop.

3. A radio frequency coupling system comprising an input circuit and an output circuit, the input circuit'comprising an inductance and a capacitance in parallel therewith, said output circuit comprising an inductance magnetically coupled to the first mentioned inductance and a capacitance, an independent inductance common to both circuits, said independent inductance being of the order of one fifteenth. of the second inductance and one five-hundredth of the first inductance.

input circuit comprising an inductance and a capacitance in parallel therewith resonating at a low frequency, said output circuit comprising an inductance magnetically coupled to the first mentioned inductance and a capacitance, an independent inductance common to both circuits, said independent inductance being of the order of one fifteenth of the second inductance and one five-hundredth of the first inductance and resonating with said input circuit at a high frequency, said output circuit includingmeansfor varying the frequency over a tuning range which is approximately centered between the aforesaid resonant frequencies of the input circuit.

5. A radio frequency coupling system comprising an input circuit and an output circuit, the input circuit comprising an inductance and a capacitance in parallel therewith, said output circuit comprising an inductance magnetically coupled to the first mentioned inductance and a capacitance, an independent inductance directly coupling both circuits, said independent inductance being in the form of a loop.

6. A radio frequency coupling system comprising an input circuit and an output circuit, the input circuit comprising an inductance and a capacitance in parallel therewith, said output circuit comprising an inductance magnetically coupled to the first mentioned inductance and a capacitance, an independent inductance directly coupling both circuits, said independent induct ance being in the form of a loop, and said output circuit comprising means for varying the frequency to which it may be tuned.

7. A radio frequency coupling system comprising an input circuit and a tuned output circuit, the input circuit comprising an inductance and a capacitance in parallel therewith, said output circuit comprising an inductance magnetically coupled to the first mentioned inductance and a capacitance, an independent inductance directly coupling both circuits, said input circuit being resonant at frequencies far removed from those to which the output circuit can be tuned, said independent inductance being in the form of a loop.

8. A multiband radio receiver including a radio frequency coupling system comprising an input circuit, a tuned output circuit, means for magnetically coupling the said circuits, inductive means consisting of loop operatively connected into both circuits and thereby directly coupling the same at least one auxiliary circuit for changing the tuning range of said output circuit to a higher frequency band, one of said circuits in-' cluding a wire for short circuiting part of said magnetically coupled means, switching means for connecting said loop into said auxiliary circuits so that said loop may optionally constitute either a low impedance device for the lowest frequency band, or a high impedance device for a higher frequency band.

9. In a radio receiver, a radio frequency coupling system comprising an input circuit including a loop inductance and a parallel resonant circuit, and a tuned output circuit including said loop and an inductance magnetically coupled to the inductive element of said parallel resonant circuit.

10; In a radio receiver, a radio frequency coupling system comprising an input circuit including a loop inductance and a parallel resonant circuit, and a tuned output circuit including said loop and an inductance magnetically coupled to the inductive element of said parallel resonant circuit, the said input circuit having resonances respectively far below the lowest frequency of the range through which the output circuit is tuned, and far above the highest frequency of said range.

11. In a radio receiver, a radio frequency coupling system comprising an input circuit including a loop inductance and a parallel resonant circuit, and a tuned output circuit including said loop and an inductance magnetically coupled to the inductive element of said parallel resonant circuit, the said input circuit having a resonant frequency of the order of one-half or less of the lowest frequency of the range through which the output circuit is tuned and having another resonant frequency of the order of two times or more the highest frequency of said range.

12. In a three band radio receiver, a radio frequency coupling system comprising an input circuit including a loop inductance and a parallel resonant circuit, auxiliary circuit means including an adjustable condenser, a short circuiting element and a coil, and switching means for connecting and disconnecting said auxiliary circuit means but maintaining said loop in circuit whereby the same may act as a low impedance loop pick-up of signals in the lower frequency band, a high impedance loop pick-up for signals in the middlefrequency band and an electrostatic pickup member for the reception of signals in the highest frequency band.

13. In a multi band radio receiver, a radiofrequency coupling system comprising an input circuit including a loop inductance and a parallel resonant circuit, a tuned output circuit includin said loop and an inductance magnetically coupled to the inductive element of said parallel resonant circuit, auxiliary circuit means including an adjustable condenser, a short circuiting element and a coil, for optional inclusion in said circuits, and switching means for connecting and disconnecting said optionally included means but maintaining said lop in both circuits whereby the same may act as a low impedance loop pickup of signals in the lowest frequency band, a high impedance loop pick-up for signals in an intermediate frequency band and an electrostatic pick-up member of signals in a higher frequency band.

JOHN DRYSDALE REID. 

